| Front page | | Contents | | Previous | | Next |

Afværge af grundvandsforurening ved kombination af Flushing og MPPE-vandrensning

Summary and conclusions

Flushing is a remediation technique that is well-suited to an aggressive treatment of the contamination source area with a non-aqueous phase liquid (NAPL) below the groundwater level. Macro Porous Polymer Extraction (MPPE) is a water treatment method which is well-suited in cases where the concentration of contaminants is high. Since flushing typically results in high concentrations, combining flushing and MPPE is a logical step.

Flushing is based on the pump-and-treat method and makes use of chemical additives (surfactants, co-solvents or chemical that form complexes) which are added to the aquifer via injection wells. The chemicals change a number of physical-chemical properties of the NAPL and promote the dissolution and/or mobilisation of the contamination resulting in the removal of the contamination from abstraction wells. The chemical additives vary in their method of action and must be selected with care.

MPPE technology makes use of small hydrophobe particles of plastic which are placed in a column. Contaminated water is pumped through the column and the contamination is removed. The ability of the particles to remove the contamination stems from an extraction liquid, which is immobilized in the pore structure of the particles. The water treatment principle is therefore based on liquid-liquid extraction. When the column is saturated with contamination, it is regenerated by way of steam treatment.

Case 1

A site of a former dry-cleaners was contaminated with tetrachloroethylene. In 1998, a pilot scale project used co-solvent flushing with ethanol in combination with water treatment with MPPE. To ensure hydraulic control, a set-up with co-solvent injection in the middel and groundwater abstraction in an outer circle was used. The ethanol concentration in the injected water was 95%. Flushing was carried out over a period of 4 days.

During the project, a total of 30-40 l PCE was removed. The concentration of PCE increased by a factor of 130 at the time of ethanol breakthrough. Since the maximum ethanol concentration in the abstracted water was approx. 25%, it was concluded that the injection of ethanol was terminated too early. The water had an average PCE concentration of 230 mg/l and <5 mg/l before and after MPPE treatment, respectively. It should be noted that ethanol was not removed by the MPPE. A conclusion from the water treatment test was that ethanol could be recycled after concentrating in order to save resources.

Case 2

The site of a former dry-cleaners in Dortmund, Germany was contaminated by PCE. In a demonstration project in 2001, a chemically-treated tenside mixture of rape seed oil, fatty acids and glycerol was used. The demonstration was carried out over a period of 16 days. The average concentration in the source area was generally 10-30 mg PCE/l prior to the demonstration.

During the demonstration period, approx. 900 m3 groundwater was abstracted. The water had an average PCE concentration of 36 mg/l and 3 mg/l before and after MPPE treatment, respectively. Nearly 29 kg PCE was removed during the demonstration project. Recovery of the tenside was 82%. Because the PCE concentration in the abstracted water did not increase after injecting the tenside, the value of the flushing was limited. The MPPE water treatment method was able to remove the PCE from the water/tenside mixture with a good efficiency.

Conclusion

Combination of flushing and MPPE is an aggressive technology that can result in significant mass reduction within a very short time period. The combination works against many different contaminants and contaminant mixtures. The method has advantages in treating contaminant source areas, where NAPL is present.

Flushing directly affects the forces that slow down the remediation of a source area with NAPL. Flushing research is expected to promote understanding of how NAPL acts in groundwater.

MPPE water treatment has functioned as intended in the investigated cases.

Disadvantages include the fact that flushing is a "course" method thatonly removes part of the contamination. Therefore, it is often necessary to supplement the method with other methods for polishing. Flushing gives a risk of spreading the contamination through mobilization. In addition, some of the chemical additive will remain in the groundwater since the recovery can be low. MPPE is not competitive in situations where the concentration of the contaminant is limited.

There are future prospects for use of the flushing/MPPE combination in Denmark since they are suitable for an initial aggressive remediation of chlorinated solvent NAPL. Few remediation alternatives exist today for the specific area.